A semiconductor slab containing multiple quantum wells is widely used in various optoelectronic devices. Examples of such semiconductors include gallium arsenide (GaAs), indium phosphate (InP), and their quaternary system of indium gallium arsenide phosphate (InxGayAs1-y P1-x). For the realization of efficient light sources such as lasers and light-emitting-diodes (LEDs), direct bandgap semiconductors are of central importance. In order to control flow of electrons and holes within such semiconductor material composites, precise control of material compositions between three or four elements is important. For example, in the case of a ternary system of AlxGa1-xAs, x is varied between 0 and 0.4 to control the bandgap size. On the other hand, in the case of InxGayAs1-y P1-x material system, both x and y are varied to get desired bandgap sizes. One such an exemplary semiconductor with a varying material composition is shown in FIG. 1. It is to be remarked that the stacked layers can be grown using molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD).
In usual optoelectronic applications, cladding layers are designed to have the larger bandgap than those of quantum wells and barriers. A proper doping element may be introduced into the cladding layer to obtain p-type or n-type semiconductor. Their combined structure with a p-n junction enables net current flow under a forward biased condition. As a result, electrons and holes can be confined within the quantum well region and their radiative recombination processes may enable efficient generation of light. In particular, high-performance semiconductor lasers may be obtained based on the wafer design very similar to the one shown in FIG. 1. Subsequent micro- and nano-fabrication procedures involving optical lithography, electron-beam lithography, and dry etching can be used to define optical waveguides and mirrors to form high-quality optical resonators.
A photonic-crystal resonator is one such an example, which can be defined on a high-refractive index semiconductor slab. As shown in FIG. 2A, a photonic-crystal resonator consists of two-dimensionally periodic arrangements of low- and high-refractive index media, where circular holes (nhole=1.0) and the semiconductor backbone (nslab˜3.5) provide a highly-reflective minor for electromagnetic waves propagating in the two dimensions. Most of the electromagnetic field's energy can be confined within the intermediate region devoid of such a periodicity, which results in the formation of an optical resonator (See FIG. 2B). This resonator may be fabricated in a semiconductor slab shown in FIG. 1. The optical gain provided by the multiple quantum wells can be used to overcome the optical loss of the resonator, thereby reaching the lasing threshold. The resulting lasing wavelength can be lithographically determined by a lattice constant, ‘a’, of the photonic-crystal minor. For example, to obtain emission wavelength of ˜1.3 μm, ‘a’ would be in the range of 300 nm-350 nm. Additional fine ‘tuning’ of the geometries (hole size & position) around the defect region also affect the emission wavelength and the optical loss of the resonator. In general, the thickness of the semiconductor slab, ‘T’, (see FIG. 1) should be chosen close to the effective half wavelength (T˜λ/2 nslab) to support only the fundamental slab guided modes. For example, for emission wavelength of ˜1.3 μm, ‘T’ would be in the range of 200 nm-250 nm.